1. Technical Field
The present invention relates to differential amplifiers and more particularly to an offset cancellation circuit which is isolated from an amplifier input and output so as not to affect impedance and loading of control loops.
2. Description of the Related Art
For high-gain differential amplifiers fabricated in integrated circuit technologies, an offset-cancellation (OC) circuit must be employed to ensure proper operation. High-gain amplifiers are typically constructed by connecting a chain of individual differential amplifier stages. The function of the OC circuit is to correct for the overall difference in gain experienced by the differential signals being amplified such that both differential signals experience a same overall gain through the amplifier chain.
Without an OC circuit, inevitable transistor-level device variations result in a small imbalance in input stages being converted to a large output offset through the high gain of the amplifier chain. The large output offset saturates the final amplifier stages, extinguishing their gain and drastically reducing the overall sensitivity of the amplifier. An OC circuit provides a method of equalizing the gain in the two differential paths to eliminate this effect.
In a typical OC implementation, the differential output signals of a high-gain amplifier are sampled and the gain or operating point of one of the first amplifier stages is adjusted to equalize the outputs. In a typical feedback implementation, an OC control loop samples and feeds back average voltages of the limiting amplifier's differential outputs to an early stage whose operating point is adjusted to minimize the difference between the average voltages of the differential outputs. In conventional implementations, an RC network with a relatively large time constant (relative to the operating data rate of the amplifier) is used to extract the DC, or average, values of the amplifier outputs. An active element in the OC loop may be included to appropriately condition the feedback signals, and the OC loop is closed by applying the feedback signals at the input of one of the early amplifier stages.
Optical receivers include functional OC control loops that must be carefully designed. In particular, at the point where feedback is applied, a loading on a preceding and a following amplifier stage must be minimized so that the stages function properly. Conventional approaches to closed loop offset cancellation include a feedback applied at an amplifier input or a feedback applied at an output of a first amplifier stage. In both cases, the feedback loop includes a resistance ROC and a capacitance COC such that only the average or DC value of the output voltages is sampled.